U.S. patent number 4,875,457 [Application Number 07/232,798] was granted by the patent office on 1989-10-24 for apparatus and method for protecting engine electronics from radio frequency interference.
This patent grant is currently assigned to Brunswick Corporation. Invention is credited to Arthur O. Fitzner.
United States Patent |
4,875,457 |
Fitzner |
October 24, 1989 |
Apparatus and method for protecting engine electronics from radio
frequency interference
Abstract
A system and method for its implementation for suppressing RFI
effects on an electronic control module enclosed in a metal housing
includes inserting a plurality of high frequency shunts in the
wires attached to the module through the housing. The shunts,
comprising by-passing capacitors include one capacitor on the
incoming wire disposed closely adjacent the housing and grounded
thereto and another capacitor on each wire disposed closely
adjacent the connection of the wire to the active circuit of the
module and grounded to the circuit ground network. The housing
includes a direct connection to ground and the ground network for
the electronic module has a single ground connection to the
housing.
Inventors: |
Fitzner; Arthur O. (Fond du
Lac, WI) |
Assignee: |
Brunswick Corporation (Skokie,
IL)
|
Family
ID: |
22874629 |
Appl.
No.: |
07/232,798 |
Filed: |
August 16, 1988 |
Current U.S.
Class: |
123/633; 333/12;
701/115; 439/607.01 |
Current CPC
Class: |
F02P
7/025 (20130101); F02P 11/00 (20130101); H04B
15/025 (20130101) |
Current International
Class: |
F02P
11/00 (20060101); F02P 7/00 (20060101); H04B
15/02 (20060101); F02P 7/02 (20060101); H04B
001/10 (); F02P 015/00 () |
Field of
Search: |
;123/633 ;333/12
;361/400 ;364/431.04,431.12,574 ;439/607 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Argenbright; Tony M.
Attorney, Agent or Firm: Andrus, Sceales, Starke &
Sawall
Claims
I claim:
1. In an electronic control module including electronic circuit
means, a metal housing and external conductors extending through
the housing and having electrical connections to the circuit means,
a system for suppressing RFI signals induced in the conductors and
entering the electronic module via the conductors comprising:
an external ground connection to the housing;
first RFI voltage shunt means connecting each conductor to the
housing closely adjacent its entry through the housing;
second RFI voltage shunt means connecting each conductor to the
ground network for the circuit means closely adjacent the
connection of the conductor to the circuit means; and,
a single ground connection between the housing and the ground
network for the circuit means.
2. The system as set forth in claim 1 wherein the first RFI voltage
shunt means comprises a non-inductive by-passing capacitor for each
conductor.
3. The system as set forth in claim 2 including a common
low-inductive RFI-intercepting ground bus connecting each capacitor
to the housing.
4. The system as set forth in claim 3 wherein the electronic
circuit means comprises a printed circuit board and the
RFI-intercepting ground bus comprises a conductive trace on the
circuit board effectively isolated from the circuit means.
5. The system as set forth in claim 2 wherein the second RFI
voltage shunt means comprises a second non-inductive by-passing
capacitor for each conductor.
6. The system as set forth in claim 5 including a high frequency
series impedance in each conductor between the first and second
shunt means.
7. The system as set forth in claim 3 wherein the RFI-intercepting
ground bus comprises a terminal strip conductively attached to the
housing, said strip including terminal means for effecting
connection of each first RFI voltage shunt means.
8. The system as set forth in claim 7 wherein the internal ground
connection to the housing comprises an intermediate connection to
the terminal strip.
9. A method of suppressing engine spark ignition RFI in an
electronic engine control module including a metal housing
enclosing the module, circuit board means within the housing, and
external conductors extending through the housing and operatively
connected to the circuit board means, the method comprising the
steps of:
(1) grounding the housing to the engine block;
(2) inserting first RFI shunt means between the housing and each
conductor closely adjacent its entry to the housing;
(3) inserting second RFI shunt means between each conductor and the
ground network for the circuit board means closely adjacent the
connection of said conductor to the circuit board means; and,
(4) establishing a single ground connection between the housing and
the ground network for the circuit board means.
10. The method as set forth in claim 9 wherein the first and second
RFI voltage shunt means each comprises a by-passing capacitor for
each of the conductors.
11. The method as set forth in claim 10 including the step of
inserting a high frequency series impedance in at least one of said
conductors between the first and second by-passing capacitors.
12. A system for suppressing the effects of engine spark ignition
RFI on an electronic module comprising:
a grounded metal housing enclosing the circuit means;
external conductors extending into the housing and connected to the
circuit means;
a plurality of high frequency shunts from each of the conductors to
ground;
one of said shunts on each conductor disposed closely adjacent the
point of entry of the conductor into the housing and conductively
attached thereto;
another of said shunts on each conductor disposed closely adjacent
the point of connection of the conductor to the circuit means and
conductively attached to the circuit means ground network; and,
a single ground connection between the housing and the circuit
means ground network.
13. The system as set forth in claim 12 wherein said housing is
grounded to the engine.
14. The system as set forth in claim 12 wherein said shunts
comprise non-inductive by-passing capacitors.
15. The system as set forth in claim 14 wherein the conductors
include at least one shielded cable and further comprising a
conductive connection from the outer shield of said shielded cable
to the housing closely adjacent the point of entry thereinto.
Description
BACKGROUND OF THE INVENTION
The present invention relates to a system for improving the RFI
noise immunity of electronic circuits and, more particularly, to a
system for shielding the engine electronics used on internal
combustion engines from spark ignition RFI, particularly where the
electronics are of necessity located in close proximity to the RFI
source.
It is well known that high voltage ignition systems are one of the
worst sources of radio frequency interference (RFI). In many
internal combustion engines, electronic control modules are
utilized to control such functions as spark ignition and fuel
injection. When utilized in compact design environments, such as
for example in outboard boat motors, the electronic control modules
are often crowded under an enclosing cowling and located
unavoidably close to a very powerful source of RFI.
For example, the second side of an ignition coil can develop 15,000
volts immediately before spark plug gap breakdown and discharge. In
some cases, the voltage may even reach levels as high as 30,000
volts. The wave front which is generated by an almost instantaneous
voltage drop from 15,000 volts to essentially zero volts results in
the radiation of electromagnetic interference signals over a wide
band. Typically, however, there is a concentration of energy at
frequencies in the range of approximately 150 megahertz. These
instantaneous bursts of high frequency RFI can induce voltages in
nearby conductors, including the wires to and from a nearby
electronic control module. Such voltage transients may be induced
in nearby wires as a result of the high currents at high
frequencies accompanying the spark discharge or the stray
capacitance coupling to a high tension lead could also cause a
voltage change in a closely positioned wire. Thus, a 15,000 volt
transient on the high tension leads of an ignition system might
easily induce a 1500 volt transient in a lead to or from an engine
electronic control module which includes the usual PC board
construction. In a typical lead wire having a typical
characteristic impedance of about 150 ohms, a 1500 volt voltage
transient will be accompanied by a 10 amp current transient and an
injection of a current of that magnitude into a circuit is likely
to cause a potential change at that point in the circuit relative
to other parts of the circuit.
It is, of course, known to use suppressor spark plugs to reduce RFI
noise from spark ignition systems. However, because suppressor
plugs may occasionally not function as intended, a single
unsuppressed firing could generate a very strong RFI signal and
cause a serious electronic circuit malfunction or failure. Also, in
high performance outboard motors using fast rise time capacitive
discharge ignition systems, the typical automotive resistor-type
suppressor spark plugs cause a drop in power output plus a
noticeable reduction in the smoothness of engine idle. For this
reason suppressor plugs are not universally used in outboard
motors.
RFI transients are particularly troublesome in digital electronic
systems. A voltage transient as low as 5 volts is normally
sufficient to change a bit in a digital system from digital zero to
one or vice versa. Furthermore, a dropped or added digital bit may
become temporarily locked in a digital circuit if it is not
immediately self-correcting. Thus, an improper logic state may
remain locked in for a significant period of time, resulting in a
control malfunction. Analog circuits are also susceptible to
malfunctions as a result of transient RFI voltages, for example,
timing circuits may be caused to operate out of proper sequence by
a transient voltage. Ordinarily, however, analog circuits tend to
be immediately self-correcting. Thus, the problems caused by RFI
are generally much more serious in a digital circuit.
Electronic modules are often shielded against RFI by enclosing them
in metal housings. Such metal housings do tend to protect the
printed circuit board of the module inside the housing from
externally generated electromagnetic fields. Nevertheless,
electronic modules used on internal combustion engines still tend
to experience problems with spark plug RFI. One of the primary
sources of trouble is in the wires entering the module through the
housing which pick up and conduct into the housing the strong
transient voltages induced from the firing of the spark plugs.
It is known to use RFI suppressing by-pass capacitors to prevent
high frequency voltage transients on the wires from reaching the
inputs of amplifiers, logic gates, etc. on the PC board inside the
housing. Typically, however, such by-pass capacitors are located to
merely shunt the RFI voltage transients directly to the ground
network on the PC board. These transient voltages are accompanied
by strong transient currents and shorting the transient voltage
directly to the circuit board ground, via a by-passing capacitor,
will result in an instantaneous injection of an electric charge at
that point. As a result, the potential at the point of injection
will also instantaneously change relative to other parts of the
ground network. Should the potential difference be high enough,
e.g. about 2.5 volts or more, a logic signal from one digital
device located at one point on the ground network may be read as at
an opposite state by another digital device located at another
point on the ground network, which is at a different relative
potential.
Among the wires typically entering the control module through the
housing is a ground wire from the engine block. Often, the ground
wire extends directly to a connection on the PC board ground
network. However, transient high frequency electrical charges
induced in the ground wire are then carried directly to the PC
board.
The metal housing itself may also experience induced voltages,
resulting in a potential difference between opposite ends of the
housing. If all four corners of the PC board inside are grounded to
the housing, as is often done in conventional design, the potential
difference across the housing could also introduce disturbances to
the PC ground network.
Also, shielded cables are frequently used to bring low level
signals into an electronic module. If the conductive shield on such
a cable is allowed to pass directly to the PC board ground network,
there will be another potential source of induced transient
voltages being transmitted directly to the PC board ground
network.
Thus, attempts to provide RFI suppression and shielding for
electronic circuit modules has often been ineffective, particularly
in high RFI environments where design requirements dictate close
proximity between the RFI source and the electronic circuitry,
either digital or analog. The generally-accepted assumption that
the ground network for a PC board is always a true system ground,
even under the varying influences of RFI transients, is believed to
be the major source of the problem.
SUMMARY OF THE INVENTION
In accordance with the present invention, a system for suppressing
induced RFI in an orderly and logical manner is presented. The
invention is based on the premise that conventional suppression
techniques which do no more than facilitate the sudden flow of
transient electrical charges into the PC board ground network are
only marginally effective.
The system of the present invention may be applied to a
conventional electronic module which includes the usual PC board or
boards, enclosed in a metal housing and to which external
conductors are connected which extend through a wall of the housing
and are connected to the PC board circuitry. The system includes an
external ground connection (for example, from the engine block)
which goes directly to the metal housing. Transient electrical
charges entering the module on the ground wire will thus first be
substantially dispersed into the housing. If the ground wire
continues on to the PC board, very little transient charge will
actually go to the PC board. Each of the other conductors entering
the housing is provided with an RFI voltage shunt (in the form of
an RFI bypass capacitor) connecting the conductor directly to the
housing closely adjacent to its entry point. Any high frequency RFI
transient voltages appearing on these conductors are thus
substantially shorted to the housing, that is, to a ground that is
essentially isolated from the ground network of the PC board or
boards comprising the electronic control module. In addition, a
second level RFI voltage shunt (in the form of a second RFI bypass
capacitor) is inserted between each conductor and the PC board
ground network at the point closely adjacent the connection of the
conductor to the PC board active circuit area itself. Any remaining
transient currents, not removed by the first RFI shunts, will be
dissipated. Finally, a single ground connection is made between the
ground network of the PC board and the housing. In this manner, any
induced potential differential from one point on the housing to
another would not introduce a corresponding disturbance in the PC
board ground network.
Each of the first and second RFI voltage shunts preferably
comprises a non-inductive by-passing capacitor. Connection between
each of the first by-passing capacitors and the housing is
preferably made via a common low-inductance RFI-intercepting ground
bus. The RFI-intercepting ground bus may be constructed
independently and conductively attached directly to the housing or
may comprise a conductive trace on the circuit board isolated from
the ground network for the active circuits and independently
grounded to the housing.
Additional isolation of the PC board from high frequency transient
current may also be provided by inserting a high frequency series
impedance in each conductor between the first and second RFI
voltage shunts, that is, between the first and second bypassing
capacitors.
Should any of the conductors entering the housing for connection to
the control module include a shielded cable, the conductive outer
shield of the cable should also be grounded directly to the housing
at the point of entry. Alternately, the cable shield may be
grounded to the RFI-intercepting ground bus.
The method of the present invention for suppressing engine spark
ignition RFI includes certain basic steps which appear to be
essential for effective suppression in high RFI environment, such
as are encountered in an outboard motor. The steps comprising the
basic method, as well as the optional additional steps, amy be
selected depending on the severity of the RFI, based on such
considerations as the proximity of the electronic control module
and the lead wires associated therewith to the RFI sources, the
lengths of the lead wires and their location and relation to one
another, etc.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a generalized schematic of an electronic control module,
showing in illustrative plan view a two-sided PC board and
incorporating the RFI suppression system of the present
invention.
FIG. 2 is a plan view similar to FIG. 1 showing the major
conductors and other key elements of the suppression system with
the top side shown in phantom to better illustrate the
complementary bottom side ground and supply traces.
FIG. 3 shows a portion of an electronic control module utilizing an
RFI-intercepting ground bus separate from the PC board.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
An electronic control module 10 includes a two-sided printed
circuit board 11 mounted inside a metal housing 12. A group of
conductors 13, comprising wires 14 or cables, extend from the
outside, through an opening 15 in a wall of the housing 12 for
eventual connection to the PC board 11. One of the wires 14
typically comprises a ground connection 16 which, in the case of an
engine control module, is attached at its outside end to the engine
block.
Each of the wires 14 entering the housing 12 through the opening 15
is provided with a first termination 17 at a point very closely
adjacent its entry into the housing. A first RFI voltage shunt 18
is connected between each first termination 17 and the housing 12.
The connection may be made directly to the housing or may be
intermediately attached via a common low-inductance
RFI-intercepting ground bus 20 which is, in turn, grounded directly
to the housing.
Each of the first RFI voltage shunts preferably comprises a first
non-inductive by-passing capacitor 21. Any high frequency RFI
transient voltages which may appear on the incoming wires 14 will
be substantially shorted out to ground via the housing 12 and
ground connection 16 which are separated from and not likely to
electrically affect the ground network or the circuitry on the PC
board 11. The connecting leads of each capacitor 21 must be kept as
short as possible to provide low inductance and the capacitor
itself must be of a non-inductive construction. The intercepting
ground bus 20, to which the ground lead of each capacitor 21 is
attached as at 22, may be a separate conductive strip or may
comprise a conductive ground trace 23 on the PC board 11. In either
case, the intercepting ground bus 20 or ground trace 23 should be
effectively isolated in a high-frequency sense from the active
circuits of the control module 10. In addition, the ground bus 20
or ground trace 23 should include multiple direct ground
connections 19 to the housing 12.
From their first terminations 17, each of the wires 14 typically
continues to the PC board 11 to a circuit connection 24 in an
active circuit trace 25 on the board. In lieu of wires 14 from the
point of first termination 17, the conductor may comprise a PC
board trace 54 forming part of an active circuit trace 25.
Very close to the connection 24 of the wire 14 (or trace 54) to the
active circuit trace 25, a second shunt 26 is connected between the
trace 25 and the ground network 27 for the circuits on the PC board
11. The ground network 27, in the embodiment shown, is shown as an
isolated network in FIG. 2, and the second shunts 26 are attached
thereto via ground network connections 28 comprising conventional
soldered leads to plated-through holes extending through the PC
board. Each of the second shunts 26 are preferably comprises a
non-inductive by-passing capacitor 30. Also, as in the case of the
first capacitors 21, short low-inductance leads should be used to
connect the capacitors 30 to the wires 14 (or traces 54) and to the
PC board ground network 27. The wires 14 or PC board traces 54
connecting the first RFI voltage shunts 18 to the second RFI
voltage shunts 26 inherently provide some series inductance. This
inherent inductance, although small in value, is useful in forcing
high-frequency transient currents entering the module on the wires
14 to flow through the bypass capacitors 21 to the housing 12 via
the RFI-intercepting ground bus 20, rather than continuing on via
the wires 14 or traces 54 into the active circuit area. To maximize
the series inductance in a simple manner, wires 14 of the smallest
practical diameter, or traces 54 of the narrowest practical width,
should be used. The first set of by-passing capacitors 21 will thus
have diverted most of the high frequency transient RFI from the
wires directly to the housing (or to the housing via the ground bus
20). Any remaining transient currents in the wires 14 (or traces
54) will be relatively small and will be diverted into the ground
network 27 by the capacitors 30. The ground network 27 should have
low resistance and low inductance to minimize local disturbances of
the potential of the ground network when the remaining small
transient currents are injected into it via the capacitors 30.
Ideally, the ground network traces should be substantially wider
than the active circuit traces and provide a gridwork or pattern
readily accessible to the ground network connections 28.
FIG. 2 shows the essentials of the grounding system and the DC
supply voltage network on the bottom side of the PC board.
RFI-intercepting ground bus 20 is shown with three
large-hole-diameter feed-through pads 50. The large diameter holes
in the feed-through pads 50 are of sufficient size to allow PC
board 11 to be attached with mounting bolts to the housing 12. The
inside walls of the holes are plated with copper or other suitable
electrically conductive material so that there is an effective
electrical connection from the RFI-intercepting ground bus 20 on
the top side of the board to an identical superimposed
RFI-intercepting ground bus 20 on the bottom side of the board.
Also, it should be understood that the small feed-through type
component mounting holes typically provided for making the ground
attachments to the capacitors 30 have not been shown in the ground
network 27. In addition, active circuit traces and circuit
components which could be included on the bottom side of the board
have been left out for clarity.
Holes 51 are provided with PC board 11 merely for mounting
purposes.
The DC ground network 27 for the active circuit area of PC board 11
is comprised of top side trace 52 and bottom side traces 53. Top
side trace 52 is itself comprised of three essentially vertical
traces and one horizontal trace, all joined into one trace 52 as
shown. The bottom side traces 53 are electrically connected to top
side trace 52 by means of feed-through connections 55 and the
large-hole-diameter feed-through at 31, thus forming a DC ground
network in the form of a grid.
The DC supply voltage network 56 is comprised of top side traces 57
and bottom side traces 58, all of which are electrically connected
into a second grid pattern by feed-through connections 59.
DC supply voltage noise-suppression capacitors 60 effectively short
the DC supply voltage grid 56 to the DC ground network grid 27 for
high frequencies. Consequently, for purposes of understanding the
RFI-suppression features of the invention, it is permissible to
assume that the two grids are merged into a single equivalent
transient voltage ground network 27 having a reduced level of
resistance and inductance for high frequency transient
currents.
The ground network 27 is connected to the housing at only one point
by means of a single mounting bolt through the large-hole-diameter
feed-through at 31. This precludes any transient potential
difference between one end of the housing and another from inducing
a potential disturbance in the PC ground network 27, which might
otherwise occur if there were multiple ground connections between
the ground network and the housing.
It is, of course, understood that for purposes of illustrative
clarity no holes other than the joining feed-through connections
have been shown in the overall grid structures of FIGS. 1 and 2.
Finally, the numerous DC supply voltage noise suppression
capacitors 60 normally used and as shown in FIG. 1, have not been
shown on FIG. 2.
Should any of the incoming wires 14 comprise a coaxial or shielded
cable, the conductive outer shield should be grounded directly to
the housing near its point of entry. Such connection may be
directly to the housing 12 or to the ground bus 20 in the same
manner as with the ground connection 16.
If the RFI transients to which the electronic control module 10 is
exposed are particularly strong, a high frequency series impedance
32 may be inserted in the wire 14 (or trace) between the capacitors
21 and 30. Such a series impedance will help to further filter out
relatively large high frequency transient currents which might
otherwise not have been adequately shunted to the ground bus 20 by
the first capacitors 21. An appropriate series impedance 32 may
comprise a ferrite bead or beads, an RF choke, a resistor, or some
combination thereof.
In FIG. 3, there is shown a portion of an electronic control module
which utilizes an RFI-intercepting ground bus that is separate from
the PC board. In those applications where it may be inconvenient or
impossible to provide a conductive ground trace 23 on the circuit
board itself, the RFI-intercepting ground bus may comprise a
separate conductive member attached directly to the housing 12.
In the embodiment shown, the intercepting ground bus 20 comprises a
separate terminal strip 33 attached to the inside wall of the
housing adjacent the opening 15 for the conductors 13. The opening
15 may be surrounded with a conventional insulating grommet 34
through which the various conductors 13 pass. The terminal strip 33
includes a conductive base 35 attached to the housing 12 with
conductive fasteners 36 and spacers 37. An insulating strip 38
extends out from the base 35 and includes a series of isolated
terminals 40.
Each of the incoming wires 14 for which it is desired to provide
RFI suppression is severed immediately after its entry into the
housing and attached to a terminal 40, as with a soldered
connection, to provide a first termination 17. The soldered
connection or the first termination 17 also provides a point for
connection of the other end of lead wire 14 and one lead 41 of a
first bypassing capacitor 21. The other lead 42 of the capacitor 21
may be conveniently attached with a soldered or welded connection
43 directly to the base 35 of the terminal strip 33. The ground
wire 16 may be soldered or welded directly to the conductive base
35 of the insulating strip at a convenient connecting point 44 or,
alternatively, could be attached to the base via one of the
fasteners 36.
RFI is not easily quantifiable nor are the effects of RFI on
digital or analog circuits easily predicted. Nevertheless, the
adverse effects of RFI on electronic circuits, particularly digital
circuits, must be eliminated or reduced to levels which the circuit
can tolerate. The system and method disclosed herein, though
incapable of precise quantitative definition, has been found in its
minimum configuration to provide fully adequate RFI suppression for
electronic circuits operating in extremely strong RFI environments,
namely, the compact environment of the spark ignition system in an
outboard motor.
* * * * *